I'm trying to create an auto complete custom field and add it to my jsd customer portal, the problem is that in my edit.vm file i have a <select></select> that work fine in the back-office but when i add the custom field to the customer portal, the custom field is displayed as a simple <input type="text"/>
I am afraid I'm not much help when it comes to modifying vm templates. However have you see this KB? How to enable Autocomplete Renderer for Multi-Select Custom Field in JIRA
It provides you with steps you can follow in order to convert an existing muli-select field custom field in Jira into a field that allows for an auto-complete aspect. I have followed this before and I know it can work with Jira, however I have not personally tested this with Service Desk. That said I still believe this can work in a customer portal that Service Desk provides.